Hi SD,
If you imagine a square wave - active low will step (a little while after) the transition from the high point of the waveform to the low point of the waveform whereas active high will step (a little while after) the transition from the low point of the waveform to the high point of the waveform.
It is a preference of some driver chips to only operate reliably when this is set correctly (if they expect to see an active low input then this is what should be used).
Tweakie.